Adaptive scheduling of voice traffic in a multi-carrier communication environment

ABSTRACT

The present invention provides a technique for scheduling data, and in particular, scheduling real-time or voice data for transmissions during a transmit time interval in a multi-carrier communication environment. For each transmit time interval, channel condition indicia for multiple users is determined, and an iterative scheduling process is then implemented based in part on the channel condition indicia. The iterative scheduling initially preassigns select tones for each of the remaining users that have not been permanently assigned tones for the given transmit time interval. Next, the remaining user having the least favorable channel conditions is selected as an active user. The newly selected active user is then permanently assigned the select tones that were initially pre-assigned to that particular user. The permanently assigned tones are removed from consideration, and the process is repeated until all the remaining users are permanently assigned unique tones for scheduling.

This application claims the benefit of U.S. provisional application Ser.No. 60/558,329 filed Mar. 31, 2004, the disclosure of which is herebyincorporated by reference in its entirety.

FIELD OF THE INVENTION

The present invention relates to wireless communications, and inparticular to scheduling voice traffic in a multi-carrier communicationenvironment.

BACKGROUND OF THE INVENTION

In orthogonal frequency division multiplexing (OFDM), the transmissionband is divided into multiple orthogonal carrier waves. Each carrierwave is modulated according to the digital data to be transmitted.Because OFDM divides the transmission band into multiple carriers, thebandwidth per carrier decreases and the modulation time per carrierincreases. Since the multiple carriers are transmitted in parallel, thetransmission rate for the digital data, or symbols, on any given carrieris lower than when a single carrier is used.

OFDM modulation requires the performance of an Inverse Fast FourierTransform (IFFT) on the information to be transmitted. A Fast FourierTransform (FFT) is used for demodulation. In practice, the IFFT and FFTare provided by digital signal processing carrying out an InverseDiscrete Fourier Transform (IDFT) and a Discrete Fourier Transform(DFT), respectively. As such, the characterizing feature of OFDMmodulation is that orthogonal carrier waves are generated for multiplebands in a simultaneous fashion within a transmission channel.

One of the primary benefits of using OFDM modulation for communicationsis to enhance the robustness of the system against selective frequencyfading and narrow band interference. Unlike single carrier systems,fading in a certain frequency range or interference in a certainfrequency range will not cause the entire communication link to fail. Inan OFDM system, only a small percentage of the parallel carriers will beaffected by fading or interference at a given frequency.

Recently, the IEEE has adopted OFDM for certain 802.11 and 802.16communication standards. These standards provide solutions for high datarate transmission in broadband systems. Although data is the currentfocus in these systems, voice services will need support in futuresystems. Unfortunately, OFDM presents certain obstacles for voiceservices. In particular, OFDM systems have large numbers of carriersthat must be assigned and allocated for different services and users. Aschannel conditions vary, carrier allocation must vary to ensureefficient use of resources, while maintaining a desired quality ofservice.

Accordingly, there is a need to control allocation and assignment ofOFDM carriers to accommodate voice traffic, while optimizing systemresources and maintaining a desired quality of service. In particular,there is a need to ensure desired transmission rates while minimizingthe number of carriers allocated to a given user.

SUMMARY OF THE INVENTION

The present invention provides a technique for scheduling data, and inparticular, scheduling real-time or voice data for transmissions duringa transmit time interval in a multi-carrier communication environment,such as an OFDM communication environment. For each transmit timeinterval, channel condition indicia for multiple users is determined,and an iterative scheduling process is then implemented based in part onthe channel condition indicia. The iterative scheduling initiallypre-assigns select OFDM tones for each of the remaining users that havenot been permanently assigned tones for the given transmit timeinterval. Next, the remaining user having the least favorable channelconditions is selected as an active user. The newly selected active useris then permanently assigned the select OFDM tones that were initiallypre-assigned to that particular user. The permanently assigned OFDMtones are removed from consideration, and the process is repeated untilall the remaining users are permanently assigned unique OFDM tones. Atthis point, scheduling may be initiated.

The OFDM tones assigned to each user may be assigned in groupscorresponding to channels. These channels define available tonesthroughout the transmit time interval. The transmit time interval isbroken into time segments, referred to as blocks, wherein all of theavailable sub-carriers in the available OFDM spectrum are repeated foreach block. Each sub-carrier in the resulting time-frequency continuumis referred to as a tone. If the tones are grouped into channels,channels may include tones over any number of frequencies or blocks.

Those skilled in the art will appreciate the scope of the presentinvention and realize additional aspects thereof after reading thefollowing detailed description of the preferred embodiments inassociation with the accompanying drawing figures.

BRIEF DESCRIPTION OF THE DRAWING FIGURES

The accompanying drawing figures incorporated in and forming a part ofthis specification illustrate several aspects of the invention, andtogether with the description serve to explain the principles of theinvention.

FIG. 1 is a block representation of a wireless communication system.

FIG. 2 is a block representation of a base station according to oneembodiment of the present invention.

FIG. 3 is a block representation of a mobile terminal according to oneembodiment of the present invention.

FIG. 4 illustrates an exemplary OFDM frame structure.

FIG. 5 illustrates the two-dimensional channels associated with an OFDMsystem over one transmit time interval.

FIG. 6 is a flow diagram illustrating the overall operation of thepresent invention according to one embodiment.

FIG. 7 illustrates assignment of OFDM tones or carriers whentransmission repetition is employed.

FIG. 8 is a flow diagram illustrating tone assignment according to oneaspect of the present invention.

FIG. 9 is a flow diagram illustrating user scheduling according to oneembodiment of the present invention.

FIG. 10 is a simplified example of scheduling according to oneembodiment of the present invention.

FIG. 11 illustrates the tones scheduled for users 1-3 according to thescheduling of FIG. 10.

FIG. 12 is a first channel structure for an OFDM system according to oneembodiment of the present invention.

FIG. 13 is a second channel structure for an OFDM system according toone embodiment of the present invention.

FIG. 14 illustrates multiple channel units to assist in reducingsignaling overhead according to one embodiment of the present invention.

FIG. 15 is a logical breakdown of an OFDM transmitter architectureaccording to one embodiment of the present invention.

FIG. 16 is a logical breakdown of an OFDM receiver architectureaccording to one embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The embodiments set forth below represent the necessary information toenable those skilled in the art to practice the invention and illustratethe best mode of practicing the invention. Upon reading the followingdescription in light of the accompanying drawing figures, those skilledin the art will understand the concepts of the invention and willrecognize applications of these concepts not particularly addressedherein. It should be understood that these concepts and applicationsfall within the scope of the disclosure and the accompanying claims.

The following description begins with an overview of a wirelesscommunication environment and the architecture of a base station, whichincludes any type of wireless or like access point for local wireless orcellular communications, and a mobile terminal. After the overview ofthe wireless communication environment, a detailed review of thescheduling techniques proposed by the current invention is provided,followed by a detailed review of exemplary transmit and receivearchitectures for facilitating orthogonal frequency divisionmultiplexing (OFDM)-based communications.

With reference to FIG. 1, a base station controller (BSC) 10 controlswireless communications within multiple cells 12, which are served bycorresponding base stations (BS) 14. In general, each base station 14facilitates communications using multi-carrier communications, such asOFDM, with mobile terminals 16, which are within the cell 12 associatedwith the corresponding base station 14. The movement of the mobileterminals 16 in relation to the base stations 14 results in significantfluctuation in channel conditions. As illustrated, the base stations 14and mobile terminals 16 may include multiple antennas to provide spatialdiversity for communications.

A high level overview of the mobile terminals 16 and base stations 14 ofthe present invention is provided prior to delving into the structuraland functional details of the preferred embodiments. With reference toFIG. 2, a base station 14 configured according to one embodiment of thepresent invention is illustrated. The base station 14 generally includesa control system 20, a baseband processor 22, transmit circuitry 24,receive circuitry 26, multiple antennas 28, and a network interface 30.The receive circuitry 26 receives radio frequency signals bearinginformation from one or more remote transmitters provided by mobileterminals 16 (illustrated in FIG. 3). Preferably, a low noise amplifierand a filter (not shown) cooperate to amplify and remove broadbandinterference from the signal for processing. Downconversion anddigitization circuitry (not shown) will then downconvert the filtered,received signal to an intermediate or baseband frequency signal, whichis then digitized into one or more digital streams.

The baseband processor 22 processes the digitized received signal toextract the information or data bits conveyed in the received signal.This processing typically comprises demodulation, decoding, and errorcorrection operations. As such, the baseband processor 22 is generallyimplemented in one or more digital signal processors (DSPs) orapplication-specific integrated circuits (ASICs). The receivedinformation is then sent across a wireless network via the networkinterface 30 or transmitted to another mobile terminal 16 serviced bythe base station 14.

On the transmit side, the baseband processor 22 receives digitized data,which may represent voice, data, or control information, from thenetwork interface 30 under the control of control system 20, and encodesthe data for transmission. The encoded data is output to the transmitcircuitry 24, where it is modulated by a carrier signal having a desiredtransmit frequency or frequencies. A power amplifier (not shown) willamplify the modulated carrier signal to a level appropriate fortransmission, and deliver the modulated carrier signal to the antennas28 through a matching network (not shown). Modulation and processingdetails are described in greater detail below.

With reference to FIG. 3, a mobile terminal 16 configured according toone embodiment of the present invention is illustrated. Similarly to thebase station 14, the mobile terminal 16 will include a control system32, a baseband processor 34, transmit circuitry 36, receive circuitry38, multiple antennas 40, and user interface circuitry 42. The receivecircuitry 38 receives radio frequency signals bearing information fromone or more base stations 14. Preferably, a low noise amplifier and afilter (not shown) cooperate to amplify and remove broadbandinterference from the signal for processing. Downconversion anddigitization circuitry (not shown) will then downconvert the filtered,received signal to an intermediate or baseband frequency signal, whichis then digitized into one or more digital streams.

The baseband processor 34 processes the digitized received signal toextract the information or data bits conveyed in the received signal.This processing typically comprises demodulation, decoding, and errorcorrection operations, as will be discussed on greater detail below. Thebaseband processor 34 is generally implemented in one or more digitalsignal processors (DSPs) and application specific integrated circuits(ASICs).

For transmission, the baseband processor 34 receives digitized data,which may represent voice, data, or control information from the controlsystem 32, which it encodes for transmission. The encoded data is outputto the transmit circuitry 36, where it is used by a modulator tomodulate a carrier signal that is at a desired transmit frequency orfrequencies. A power amplifier (not shown) will amplify the modulatedcarrier signal to a level appropriate for transmission, and deliver themodulated carrier signal to the antennas 40 through a matching network(not shown). Further detail of OFDM transmission and receptionarchitectures are provided at the end of the specification, althoughthose skilled in the art will appreciate that the techniques disclosedherein are applicable to any multi-carrier communication environment.

As noted, OFDM modulation divides the transmission band into multipleorthogonal carrier waves, which are transmitted in parallel. Using anIFFT process, each carrier wave is modulated according to the digitaldata to be transmitted on that particular carrier wave. Since themultiple carriers are transmitted in parallel, the transmission rate forthe digital data, or symbols, on any given carrier is lower than when asingle carrier is used.

In OFDM communications, packets arriving at a base station 14 forforwarding over a forward link to the mobile terminal 16 are transmittedin OFDM frames. FIG. 4 illustrates an exemplary frame structure for OFDMcommunications. In the illustrated frame structure, each OFDM frame hasa duration of 20 ms, and consists of ten transmit time intervals (TTIs)having a 2 ms duration. The voice packets may be assigned to any one ormore TTIs, depending on channel conditions. The voice packet may beplaced entirely into one TTI or broken into multiple sub-packets anddelivered in multiple TTIs. Each TTI is formed by six blocks, each ofwhich has two OFDM symbols. The transmission blocks have a 0.333 msduration, while the OFDM symbols within a given block have a 0.1667 msduration. The OFDM symbol will generally include a preamble portion anda data portion.

As noted, OFDM provides a two-dimensional transmission system wherein atany given time multiple sub-carriers, hereafter referred to as “tones,”are used to carry relatively lower rate data in parallel. FIG. 5illustrates this two-dimensional architecture over one TTI. There aresix blocks (B₁-B₆) throughout the TTI in the time domain, and there areF_(M) OFDM tones in the frequency domain. Accordingly, there are F_(M)tones that can be used for transmission during each time block. Withinthis architecture, packets to be transmitted to the mobile terminal 16during a given TTI may be transmitted using any number of the tones inone or more of the blocks. As such, the base station 14 must assignvarious tones over the various time blocks in each transmit timeinterval to one or more mobile terminals 16 to facilitate scheduling.The present invention provides an efficient and effective system forproviding scheduling when at least some of the packets are carryingvoice or other real-time data.

With reference to FIG. 6, a flow diagram provides an overview of thebasic operation of the present invention according to one embodiment.The process of scheduling tones for various users over a given TTIstarts (step 600) by obtaining channel condition information for eachuser (step 602). The channel condition information is indicative of thechannel conditions for each tone in the OFDM channel for each user.Although actual measurements for each tone may not be taken, the channelcondition information allows for estimates to be made for all the tones,select tones, or groups of tones, sufficient to allow for assigningtones to the respective users based on relative channel conditions.

Once channel information is determined for the upcoming TTI, availabletones are pre-assigned to the active users based on the channelinformation (step 604). In general, the active users are pre-assignedthe tones associated with the best channel conditions. In particular,the tones are pre-assigned in a manner wherein the number of tonesassigned to each user is minimized while maintaining a sufficient numberof tones to ensure a desired data rate.

Next, the active user having the worst channel conditions is selected(step 606) and the pre-assigned tones are then permanently assigned tothe selected user (step 608). The permanently assigned tones may be thesame or different tones, and may be distributed throughout the timeblocks within the given TTI. Assuming there are remaining tones thathave not been permanently assigned, the process repeats if there are anyremaining users (step 610). Since this was the first time through, thereare likely additional users, and therefore, these remaining users needto be assigned tones for transmission within the TTI. The availabletones that have not been permanently assigned to other users are againpre-assigned to the remaining users based on the channel information(step 604). The remaining user having the worst channel conditions isselected (step 606), and the pre-assigned tones are permanently assignedto the selected user (step 608). The process will repeat for the TTIuntil all of the active users have been permanently assigned tones fortransmission. Once all users have been assigned tones, scheduling forthe next TTI is initiated (step 612), wherein channel conditioninformation for each of the active users in the new TTI will be obtained(step 602) prior to assigning tones to the active users as describedabove.

As is evident from the above, obtaining channel condition informationfor each user is necessary for scheduling. In one embodiment, thecarrier-to-interference ratio (CIR) for each tone is either measured orestimated based on channel condition information reported by the users,or in particular, by the mobile terminals 16. The CIR for each user maybe calculated by averaging the instant CIRs over each of the blocks in acorresponding TTI. This results in a reasonable CIR reporting intervalcorresponding to a TTI. Given the inherent delays in providing feedbackfor channel conditions, there is a certain delay in CIR reporting thatmust be taken into account. In practice, the average CIR over the sixblocks in the TTI for select tones may be reported, and the CIR for eachtone may be calculated by using linear interpolation. Further, groups oftones over one or more blocks may be associated with a single CIR toreduce signaling overhead. Such details are provided further below.

Further details are now provided with respect to one embodiment of thepresent invention. As stated above, the remaining users to be scheduledare prioritized based on their respective CIRs over the tones and blockswithin the TTI. Assuming there are n available tones associated with anestimated CIR, the remaining users are sorted in ascending order interms of their reported CIR, which is represented by Γ_(n,k), n=1, 2, .. . N for the k^(th) user. Further assume that the maximum number ofavailable tones for each TTI would be equal to the number of OFDM tonesN_(f) multiplied by the number of time blocks N_(B). During poor channelconditions, certain voice packets or real-time packets may need to besent repeatedly, or multiple times within the TTI. As such, if there areN_(u,k) tones assigned for each user, there are only N_(R,k) tones usedfor each original voice packet, and the remaining tones are used forrepetition to enhance robustness of the transmission. The tones used forrepetition are represented by M_(C,k). Thus, for the k^(th) user, therelationship between n_(u,k) and n_(R,k) isn_(u,k)=n_(R,k)*M_(C,k)+m_(k), where m_(k) represents supplemental tonesused for repetition. This relationship is illustrated in FIG. 7. Asdepicted, as the number of tones used for voice transmission increases,the data rate will increase. Further, as the number of tones used forrepetition (M_(R,k)) increases, the robustness of the transmission isincreased and will effectively enhance the CIR for a given user.

During the scheduling process, the voice payload transmitted in eachframe may be kept relatively constant with a target rate of R_(TARGET).Further, the minimum payload for each TTI transmission may be set toR_(MIN). As such, the scheduler may prevent the transmission of a packetwhose payload size is less than the minimum target rate R_(n). Further,the maximum number of tones may be assigned to a given user N_(MAX), andmay be limited when the reported CIR for the tones for a given user isextremely low and transmission repetition would be excessively high.

In order to optimize tone assignment for the kth user, the presentinvention attempts to minimize the number of tones used by a given userwhile keeping the payload slightly larger than or equal to thepredetermined payload needed for voice transmission in each frame. Thus,the optimization problem for tone assignment has the following form:

$\begin{matrix}\begin{matrix}\min & N_{U,k} \\M_{R,k} & \; \\{{subject}\mspace{14mu}{to}} & {{\overset{\_}{\Gamma}{k\left( {N_{U,k},M_{R,k}} \right)}} = {\frac{1}{M_{R,k}}*{\sum\limits_{n = 1}^{N_{U,k}}\Gamma_{n,k}}}} \\\; & {{f\left( {\overset{\_}{\Gamma}\; k} \right)} = R_{k}} \\\; & {{M_{R,k}*R} \geq R_{MIN}} \\\; & \left\{ \begin{matrix}{{M_{R,k}*R} > R_{TARGET}} & {if} & {N_{U,k} < N_{MAX}} \\{{M_{R,k}*R} \leq R_{TARGET}} & {if} & {N_{U,k} = N_{MAX}}\end{matrix} \right. \\{for} & {{N_{U,k} = 1},2,\ldots\mspace{11mu},{{N\mspace{14mu}{and}\mspace{14mu} M_{R,k}} = 1},2,\ldots\mspace{11mu},N_{U,k}}\end{matrix} & {{Eq}.\mspace{14mu} 1}\end{matrix}$where ƒ(x) is a mapping function of a data rate, which may be equivalentto or at least correspond to the payload transmitted in each TTI inlight of various channel conditions.

The optimization problem depicted in Equation 1 is a non-linear formulathat may be simplified using the mapping function from a link levelcurve, which is obtained by the relationship between block error ratesand the signal-to-noise ratios for different coding rates and modulationtechniques. From Equation 1, the number of tones used for original(non-repetition) traffic can be taken as large as possible in an effortto minimize the total number of tones used by a particular user N_(U,k).Assume that N is the number of remaining tones after assigning a certainnumber of active users. Further assume that N_(MIN,k) is the minimumnumber of tones required for user k's transmission due to the minimumCIR limitation, defined as N_(M,k) is equal to the CIR threshold for theworst link level curve, divided by maximum reported CIR over all tonesor groups of tones for user k. With this information, tone assignmentfor the k^(th) user may be provided according to the tone assignmentprocess illustrated in FIG. 8.

The process begins (step 800) by determining whether any remaining usersneed scheduling (step 802). Assuming that there are remaining users thatrequire scheduling, the reported CIRs associated with the selected user(k) in all available tones or tone groups are sorted in ascending order(step 804). Next, the minimum number of tones required for the selecteduser's transmission is determined (step 806). If the number of remainingunassigned tones (N) is greater than or equal to the minimum number oftones required for transmission (N_(MIN,k)) (step 808), the processcontinues wherein the total number of tones used for transmission foruser k (N_(U,k)) is set equal to the minimum number of tones requiredfor transmission (N_(MIN,k)) in an effort to minimize the number oftones used for transmitting voice for user k (step 810). Next, thenumber of tones transmitting original data (non-repetition data)(M_(R,k)) is set equal to the total number of tones used fortransmission by user k (N_(U,k)) (step 812). At this point, N_(R,k) andN_(U,k) equal N_(M,k), which is the minimum number of tones required fortransmission. N_(M,k) may be a fixed number for all users, or for theselected user k.

Next, the process determines whether the average CIR (or other CIRmeasurement) meets the target data rate for transmission (step 814). Ifthe average CIR meets the target data rate, the minimum number of tonesrequired for transmission (N_(M,k)) is sufficient for transmission, andthe process ends, wherein the total number of tones used fortransmission for user k (N_(U,k)) having the best CIR are pre-assignedto user k (step 816) and the process repeats for the next user.

If the average CIR does not meet the target data rate required fortransmission (step 814), this indicates that the absolute minimum numberof tones required for transmission to meet data rates (N_(M,k)) is notsufficient for transmission in light of channel conditions. In oneembodiment, when the average CIR does not meet the target data rate, thenumber of tones transmitting original data (non-repetition data) isdecreased in an effort to allow the average CIR for the tones used fortransmission to meet the target data rate. Thus, the number of tonesused for transmitting original data is decremented until the average CIRmeets the target data rate (steps 818 and 820). If the number of tonesfor transmitting original data is decremented to zero and the averageCIR is still not met given the number of tones used for transmission forthe user k (N_(U,k)), the number of tones used for transmission for userk (N_(U,k)) is incremented (step 822). Assuming the number of tones usedfor transmission for user k does not exceed the maximum number of tonesallowed for transmission (N_(MAX)) (step 824), the number of carriertones transmitting original data (M_(R,k)) is set equal to theincremented number of tones used for transmission for user k (N_(U,k))(step 812).

At this point, there is another check to determine whether the averageCIR meets the target data rate for the new number of tones used fortransmission for user k (step 814). If the average CIR still does notmeet the target data rate, the process repeats by decrementing thenumber of tones used for transmitting original data (M_(R,k)) untilM_(R,k)=0. Then, the total number of tones used for transmission foruser k (N_(U,k)) is incremented as described above. This processcontinues until the average CIR in light of the number of tones used fortransmission for user k (N_(U,k)) is sufficient to meet the target datarate. Once the target data rate can be met, the tones or groups of toneshaving the best CIRs are pre-assigned to user k (step 816). Inparticular, the best N_(U,k) tones (the number of tones used fortransmission for user k) are pre-assigned to user k. Once the tones arepre-assigned to all users in the TTI, the process ends (step 826). Basedon the above, the best available tones for user k have been pre-assignedbased on the pertinent channel information in a manner minimizing thenumber of tones pre-assigned to user k while maintaining a desired datarate.

Turning now to FIG. 9, an exemplary process for user selection once allthe active users have been pre-assigned tones is provided. The processbegins (step 900) when tone assignment is initiated after each activeuser has been pre-assigned tones for a given TTI (step 902). Ascheduling factor is determined for each of the active users (step 904).The scheduling factor may take many forms, such as a minimum reportedCIR (Γ_(MIN,k)) or an average CIR ( Γ _(k)). Next, the user with theminimum scheduling factor is selected (step 906). The minimum schedulingfactor represents the user with the worst channel conditions, and mostlikely with the largest number of tones used for transmission (N_(U,k)).Next, the previously pre-assigned tones for the selected user areassigned to the selected user (step 908). These permanently assignedtones will no longer be available for assigning to the remaining usersfor the current TTI. Thus, the available tones for the next user'sscheduling are then determined (step 910). Next, the process determineswhether there are any remaining tones available for user transmission(step 912). If there are remaining tones, the process determines whetherthere are any remaining users requiring scheduling (step 914). If thereare remaining tones for transmission and remaining users requiringscheduling, the process will repeat for the remaining users. If thereare not any remaining tones for transmission (N) or no other users needscheduling, the process ends for the current TTI (step 916).

With reference to FIG. 10, an example of the above process isillustrated. From the above, the present invention pre-assigns anoptimum number of tones for each remaining user based on channelconditions to achieve a desired data rate over a given TTI. Differentusers may be pre-assigned different or the same tones, as well asdifferent numbers of tones, depending on channel conditions. The exampleprovided in FIG. 10 assumes that there are three active users, eightOFDM tones for each symbol, and six blocks for each TTI. An ‘O’designates a tone pre-assignment, whereas an ‘X’ indicates a permanentlyassigned tone. The procedure for scheduling the three active usersfollows.

When scheduling the first user, all tones for all blocks are available.As such, there are effectively 48 channels for a given TTI. Each channelis referenced as CH_(f,b), wherein f represents a tone index and brepresents a block index for the respective channel. Assume that thechannel information for each of the users and each of the tones havebeen determined, such that each channel has a corresponding schedulingfactor for each user. Further assume that the pre-assignment processdictates that channels have been pre-assigned for users #1, #2, and #3as illustrated in the top row. In particular, User #1 has beenpre-assigned channel CH_(2,1), CH_(2,2), CH_(2,3), CH_(2,4), andCH_(2,5). User #2 has been pre-assigned channels CH_(3,1), CH_(3,2),CH_(3,3), CH_(3,4), CH_(3,5), CH_(3,6), CH_(2,1), and CH_(2,2). User #3has been pre-assigned channels CH_(6,1), CH_(6,2), CH_(6,3), andCH_(6,4). Based on the pre-assignment information, the user with theminimum scheduling factor, User #2, is selected as the first active userand the channels initially pre-assigned to User #2 are permanentlyassigned to User #2 for the given TTI. As such, in the second row ofFIG. 10, the channels pre-assigned to User #2 (CH_(3,1), CH_(3,2),CH_(3,3), CH_(3,4), CH_(3,5), CH_(3,6), CH_(2,1), and CH_(2,2)) arepermanently assigned to User #2 and are not available for User #1 orUser #3. As such, these channels are indicated as being permanentlyassigned with an ‘X.’

The scheduling process continues by providing the pre-assignment processfor User #1 and User #3 given the remaining channels. Assume that thesecond round of the pre-assignment process pre-assigns User #1 channelsCH_(2,3), CH_(2,4), CH_(2,5), CH_(2,6), CH_(1,1), and CH_(1,2) andpre-assigns User #3 channels CH_(6,1), CH_(6,2), CH_(6,3), and CH_(6,4).Again, the remaining user having the minimum scheduling factor isselected. The remaining user having the minimum scheduling factor isUser #1, and thus the pre-assigned channels for User #1 are permanentlyassigned to User #1 for the given TTI. Thus, channels CH_(3,1),CH_(3,2), CH_(3,3), CH_(3,4), CH_(3,5), CH_(3,6), CH_(2,1), CH_(2,2),CH_(2,3), CH_(2,4), CH_(2,5), CH_(2,6), CH_(1,1), and CH_(1,2) are nowpermanently assigned to active users User #1 and User #2. User #3 is theonly remaining user. The pre-assignment process is again provided forthe remaining user, User #3, wherein channels CH_(6,1), CH_(6,2),CH_(6,3), and CH_(6,4) are pre-assigned to User #3, and User #3 becomesthe final active user. The pre-assigned channels are then permanentlyassigned to User #3 for the TTI. At this point, all of the active usershave been assigned an optimal number of tones in light of the schedulingcriteria.

With reference to FIG. 11, the permanently assigned channels for activeusers User #1, User #2, and User #3 are highlighted. The channels, ortones, used for transmission are CH_(3,1), CH_(3,2), CH_(3,3), CH_(3,4),CH_(3,5), CH_(3,6), CH_(2,1), CH_(2,2), CH_(2,3), CH_(2,4), CH_(2,5),CH_(2,6), CH_(1,1), CH_(1,2), CH_(6,1), CH_(6,2), CH_(6,3), andCH_(6,4). In the above example, each channel corresponds to an OFDMtone.

During transmission, signaling must be provided between the transmitterand receiver not only to provide channel condition information, but tocommunicate the channel assignments for a given TTI for each user. Ifthe total number of OFDM tones is large, which is normally the case, thesignaling between the transmitter and receiver can become too complexand time consuming. As such, one embodiment of the present inventiondefines a unit channel, which is assigned to multiple tones over thetone-block continuum of a TTI. Thus, a unit channel consists of multipleOFDM tones and multiple blocks, resulting in a two-dimensional channelassigned for voice or other real-time transmissions. User informationmay be transmitted on a single unit channel or on multiple unitchannels, depending on channel conditions. Assignment and pre-assignmentof unit channels may take place as described above. The unit channel maytake any structure as long as it can be easily pre-determined betweenthe transmitter and receiver and referenced during signaling. FIGS. 12and 13 illustrate different unit channel configurations.

In FIG. 12, each unit channel consists of twelve OFDM tones over thefrequency and time domain of the TTI. As illustrated, each channelcovers two adjacent tones over each of the six time blocks. As such, thetotal number of unit channels (CH_(N)) is equal to half the total numberof tones (F_(M)). In FIG. 13, there are eight unit channels, each formedby many OFDM tones that are substantially uniformly spread over theentire frequency and time domain of the TTI. As such, when one or moreunit channels are assigned to a user, transmission will occur over tonesspread throughout the time-frequency continuum of the TTI. Those skilledin the art will recognize other techniques for configuring unitchannels. For the illustrated examples, the unit channel configurationof FIG. 13 may be appropriate where users may move at a high velocity,which generally causes channel conditions to change quickly. On such achannel, it is generally better to spread the tones assigned to the unitchannel over the entire frequency and time domain to achieve greaterfrequency diversity. In situations with relatively low user velocity,there is a greater emphasis on providing user diversity while keepingthe tones assigned to a unit channel relatively clustered together asillustrated in FIG. 12.

To assign one or more unit channels to a particular user, signalinginformation that is provided in control signals must be sent to eachuser for each TTI. Given the need to efficiently provide such signalinginformation to each of the users, there is a need to minimize the amountof signaling required to alert the users of the assigned channels. Inone embodiment of the present invention, a multiple unit channeladdressing technique is used. This technique assigns different sizedunit channels depending on channel conditions. Thus, if a userexperiences a low CIR, larger sized unit channels are used. If a user isexperiencing a higher CIR, smaller unit channels are used.

FIG. 14 illustrates four unit channel configurations for allocating 64channels into different sized unit channels. In the top leftconfiguration, each unit channel corresponds to an actual channel ortone. In the top right, each unit channel corresponds to two actualchannels for a total of 32 unit channels. In the bottom left, each unitchannel corresponds to four actual channels for a total of 16 unitchannels. In the bottom right, each unit channel corresponds to eightactual channels for a total of eight unit channels. For users having avery high CIR, the configuration with 64 unit channels may be used. Forusers having a high CIR, the configuration with 32 unit channels may beused. For users having a low CIR, the configuration having 16 unitchannels may be used. For users with a very low CIR, the configurationhaving eight unit channels may be used.

For any given TTI, different sized channels may be used to optimizechannel allocation and signaling requirements. For example, one largeand one small unit channel may be assigned to a given user to minimizesignaling overhead as well as efficiently assign the most appropriatenumber of channels. When unit channels are used for transmission,certain tones within a channel may not be used during certain TTIs, ifthe number of actually assigned tones does not correlate with an exactnumber of actual channels and the number of assigned unit channels.

Due to the delay constraints for real-time services, and in particularfor voice transmissions, the transmission or retransmission ofinformation must be accomplished within a short timeframe as compared todata transmissions, which are not time-sensitive. In particular,different situations may require an attempt to increase the robustnessof retransmissions as well as original transmissions. For example,retransmission robustness may need to be increased as originaltransmissions or earlier retransmission attempts fail. Further, if thereare indications of short-term frame error rates being too high, originaltransmission robustness may need to be increased by replicatingtransmissions within one or more TTIs. One embodiment of the presentinvention takes into account two flexible CIR margins. The first CIRmargin depends on the number of retransmissions within a frame, whilethe other depends on the number of frame errors over a given window ofmultiple frames. For example, the window may be a 400 ms window, whichis capable of including 20 ms frames.

For the first CIR margin depending on the number of retransmissionswithin a frame, an adaptive CIR margin increases as the number ofretransmissions increases. The adaptive margin for a user k can berepresented as:Δ(n _(k) ^((ReTx)))=ƒ_(MARGIN)(n _(k) ^((ReTx)))  Eq. 2where n_(k) ^((ReTx)) is the number of retransmissions for user k, andf_(MARGIN) is the margin function, which could either be a linear or aconcave increment function.

For an adaptive CIR margin based on frame errors, the CIR margin valuemay increase as the number of frame errors increases. The adaptivemargin can be represented as:

$\begin{matrix}{{\Delta\left( n_{k}^{({FE})} \right)} = \left\{ \begin{matrix}{f_{MARGIN}\left( n_{k}^{({FE})} \right.} & {{{if}\mspace{14mu} n_{k}^{({FE})}} < \eta} \\0 & {Otherwise}\end{matrix} \right.} & {{Eq}.\mspace{14mu} 3}\end{matrix}$where n_(k) ^((FE)) is the accumulated number of frame errors updatedevery 400 ms window, and f_(MARGIN) is the margin function, which may beeither a linear or a concave increment function. In addition, η is thenumber of frame errors allowed to happen over a window without anyimpact on voice performance.

With reference to FIG. 15, a logical OFDM transmission architecture isprovided according to one embodiment. Initially, the base stationcontroller 10 will send data to be transmitted to various mobileterminals 16 to the base station 14. The base station 14 may use channelinformation and other scheduling criteria associated with the mobileterminals 16 to schedule the data for transmission as well as selectappropriate coding and modulation for transmitting the scheduled data44. The channel information and scheduling information may be directlyfrom the mobile terminals 16 or determined at the base station 14 basedon information provided by the mobile terminals 16.

The scheduled data 44, which is a stream of bits, is scrambled in amanner reducing the peak-to-average power ratio associated with the datausing data scrambling logic 46. A cyclic redundancy check (CRC) for thescrambled data is determined and appended to the scrambled data usingCRC adding logic 48. Next, channel coding is performed using channelencoder logic 50 to effectively add redundancy to the data to facilitaterecovery and error correction at the mobile terminal 16. The channelencoder logic 50 uses known Turbo encoding techniques in one embodiment.The encoded data is then processed by rate matching logic 52 tocompensate for the data expansion associated with encoding.

Bit interleaver logic 54 systematically reorders the bits in the encodeddata to minimize the loss of consecutive data bits. The resultant databits are systematically mapped into corresponding symbols depending onthe chosen baseband modulation by mapping logic 56. Preferably,Quadrature Amplitude Modulation (QAM) or Quadrature Phase Shift Key(QPSK) modulation is used. The degree of modulation is preferably chosenbased on a channel quality indicator (CQI) for the particular mobileterminal 16. The symbols may be systematically reordered to furtherbolster the immunity of the transmitted signal to periodic data losscaused by frequency selective fading using symbol interleaver logic 58.

At this point, groups of bits have been mapped into symbols representinglocations in an amplitude and phase constellation. When spatialdiversity is desired, blocks of symbols are then processed by space-timeblock code (STC) encoder logic 60, which modifies the symbols in afashion making the transmitted signals more resistant to interferenceand more readily decoded at a mobile terminal 16. The STC encoder logic60 will process the incoming symbols and provide n outputs correspondingto the number of transmit antennas 28 for the base station 14. Thecontrol system 20 and/or baseband processor 22 will provide a mappingcontrol signal to control STC encoding. At this point, assume thesymbols for the n outputs are representative of the data to betransmitted and capable of being recovered by the mobile terminal 16.See A. F. Naguib, N. Seshadri, and A. R. Calderbank, “Applications ofspace-time codes and interference suppression for high capacity and highdata rate wireless systems,” Thirty-Second Asilomar Conference onSignals, Systems & Computers, Volume 2, pp. 1803-1810, 1998, which isincorporated herein by reference in its entirety.

For the present example, assume the base station 14 has two antennas 28(n=2) and the STC encoder logic 60 provides two output streams ofsymbols. Accordingly, each of the symbol streams output by the STCencoder logic 60 is sent to a corresponding IFFT processor 62,illustrated separately for ease of understanding. Those skilled in theart will recognize that one or more processors may be used to providesuch digital signal processing, alone or in combination with otherprocessing described herein. The IFFT processors 62 will preferablyoperate on the respective symbols to provide an inverse FourierTransform. The output of the IFFT processors 62 provides symbols in thetime domain. The time domain symbols are grouped into frames, which areassociated with a prefix by like insertion logic 64. Each of theresultant signals is up-converted in the digital domain to anintermediate frequency and converted to an analog signal via thecorresponding digital up-conversion (DUC) and digital-to-analog (D/A)conversion circuitry 66. The resultant (analog) signals are thensimultaneously modulated at the desired RF frequency, amplified, andtransmitted via the RF circuitry 68 and antennas 28. Notably, pilotsignals known by the intended mobile terminal 16 are scattered among thesub-carriers or tones. The mobile terminal 16, which is discussed indetail below, may use the pilot signals for channel estimation.

Reference is now made to FIG. 16 to illustrate reception of thetransmitted signals by a mobile terminal 16. Upon arrival of thetransmitted signals at each of the antennas 40 of the mobile terminal16, the respective signals are demodulated and amplified bycorresponding RF circuitry 70. For the sake of conciseness and clarity,only one of the two receive paths is described and illustrated indetail. Analog-to-digital (A/D) converter and down-conversion circuitry72 digitizes and downconverts the analog signal for digital processing.The resultant digitized signal may be used by automatic gain controlcircuitry (AGC) 74 to control the gain of the amplifiers in the RFcircuitry 70 based on the received signal level.

Initially, the digitized signal is provided to synchronization logic 76,which includes coarse synchronization logic 78, which buffers severalOFDM symbols and calculates an auto-correlation between the twosuccessive OFDM symbols. A resultant time index corresponding to themaximum of the correlation result determines a fine synchronizationsearch window, which is used by fine synchronization logic 80 todetermine a precise framing starting position based on the headers. Theoutput of the fine synchronization logic 80 facilitates frameacquisition by frame alignment logic 84. Proper framing alignment isimportant so that subsequent FFT processing provides an accurateconversion from the time to the frequency domain. The finesynchronization algorithm is based on the correlation between thereceived pilot signals carried by the headers and a local copy of theknown pilot data. Once frame alignment acquisition occurs, the prefix ofthe OFDM symbol is removed with prefix removal logic 86 and resultantsamples are sent to frequency offset correction logic 88, whichcompensates for the system frequency offset caused by the unmatchedlocal oscillators in the transmitter and the receiver. Preferably, thesynchronization logic 76 includes frequency offset and clock estimationlogic 82, which is based on the headers to help estimate such effects onthe transmitted signal and provide those estimations to the correctionlogic 88 to properly process OFDM symbols.

At this point, the OFDM symbols in the time domain are ready forconversion to the frequency domain using FFT processing logic 90. Theresults are frequency domain symbols, which are sent to processing logic92. The processing logic 92 extracts the scattered pilot signal usingscattered pilot extraction logic 94, determines a channel estimate basedon the extracted pilot signal using channel estimation logic 96, andprovides channel responses for all sub-carriers using channelreconstruction logic 98. In order to determine a channel response foreach of the sub-carriers, the pilot signal is essentially multiple pilotsymbols that are scattered among the data symbols throughout the OFDMsub-carriers in a known pattern in both time and frequency.

The processing logic compares the received pilot symbols with the pilotsymbols that are expected in certain sub-carriers at certain times todetermine a channel response for the sub-carriers in which pilot symbolswere transmitted. The results are interpolated to estimate a channelresponse for most, if not all, of the remaining sub-carriers for whichpilot symbols were not provided. The actual and interpolated channelresponses are used to estimate an overall channel response, whichincludes the channel responses for most, if not all, of the sub-carriersin the OFDM channel.

The frequency domain symbols and channel reconstruction information,which are derived from the channel responses for each receive path areprovided to an STC decoder 100, which provides STC decoding on bothreceived paths to recover the transmitted symbols. The channelreconstruction information provides equalization information to the STCdecoder 100 sufficient to remove the effects of the transmission channelwhen processing the respective frequency domain symbols

The recovered symbols are placed back in order using symbolde-interleaver logic 102, which corresponds to the symbol interleaverlogic 58 of the transmitter. The de-interleaved symbols are thendemodulated or de-mapped to a corresponding bitstream using de-mappinglogic 104. The bits are then de-interleaved using bit de-interleaverlogic 106, which corresponds to the bit interleaver logic 54 of thetransmitter architecture. The de-interleaved bits are then processed byrate de-matching logic 108 and presented to channel decoder logic 110 torecover the initially scrambled data and the CRC checksum. Accordingly,CRC logic 112 removes the CRC checksum, checks the scrambled data intraditional fashion, and provides it to the de-scrambling logic 114 forde-scrambling using the known base station de-scrambling code to recoverthe originally transmitted data 116.

In parallel to recovering the data 116, a channel quality indicator(CQI) corresponding to channel conditions is determined and transmittedto the base station 14. The CQI may be a function of thecarrier-to-interference ratio (CIR), as well as the degree to which thechannel response varies across the various sub-carriers in the OFDMfrequency band. The channel gain for each sub-carrier in the OFDMfrequency band being used to transmit information may be comparedrelative to one another to determine the degree to which the channelgain varies across the OFDM frequency band. Although numerous techniquesare available to measure the degree of variation, one technique is tocalculate the standard deviation of the channel gain for eachsub-carrier throughout the OFDM frequency band being used to transmitdata.

Continuing with FIG. 16, a relative variation measure may be determinedby providing the channel response information from the channelestimation function 96 to a channel variation analysis function 118,which will determine the variation and channel response for each of thesub-carriers in the OFDM frequency band, and if standard deviation isused, calculate the standard deviation associated with the frequencyresponse. Once the channel variation analysis is provided, a variationmeasure is provided to a CQI function 120 or to the baseband processor34 for transmission back to the base station 14 via the transmitcircuitry 36, depending on the configuration of the embodiment. If theCQI is determined at the base station 14, then the mobile terminal 16will provide information indicative of the CIR as well as the variationanalysis to the base station 14, which will calculate a CQI and controlscheduling as well as coding and modulation for subsequent transmissionsto the mobile terminal 16. If the CQI is generated at the mobileterminal 16 and transmitted to the base station 14, the CQI function 120will receive a CIR from a CIR function 122 and will use the CIR and thevariation measurement to either calculate or look up through a look-uptable an appropriate CQI, which is then transmitted to the base station14 via the transmit circuitry 36.

The CIR function 122 will preferably receive channel responseinformation from the channel estimation function 96 and determine theCIR based on the relative strengths of the desired carrier in light ofother interferers in traditional fashion. When pilot symbols are passedthrough the channel estimation function 96, the pilot symbols arefiltered in a manner exploiting the known pilot symbols to remove noiseand interference. The output of the channel estimation function 96 isintended to be a noiseless replica of the pilot symbol. With thisreplica, the carrier power may be determined, as well as subtracted fromthe received pilot symbol to yield a noise plus interference signal.This resulting signal is computed to provide an interference power,which is compared to the carrier power to determine the CIR. One exampleof determining a CIR is provided in co-assigned U.S. patent applicationSer. No. 10/038,916 filed Jan. 8, 2002, which is incorporated herein byreference in its entirety. Those skilled in the art will recognizenumerous techniques for determining the CIR, and if desired, calculatingCQI.

Those skilled in the art will recognize improvements and modificationsto the preferred embodiments of the present invention. All suchimprovements and modifications are considered within the scope of theconcepts disclosed herein and the claims that follow.

1. A method for scheduling data for transmission during a transmit timeinterval in a multi-carrier communication environment comprising:determining channel condition indicia for a plurality of users; in aniterative manner: pre-assigning select tones for each remaining user ofthe plurality of users that has not been permanently assigned tones forthe transmit time interval; selecting a remaining user having leastfavorable channel conditions as an active user; and permanentlyassigning to the active user the select tones pre-assigned to the activeuser, wherein once the select tones are permanently assigned to theactive user, the active user is no longer a remaining user.
 2. Themethod of claim 1 wherein the select tones permanently assigned toactive users are no longer available for pre-assignment to the remainingusers.
 3. The method of claim 1 further comprising initiating schedulingfor the transmit time interval for the plurality of users using theselect tones permanently assigned to each of the plurality of users. 4.The method of claim 1 wherein for each remaining user, pre-assigning theselect tones comprises: sorting tones in light of channel conditioninformation; and selecting ones of the tones having most favorablechannel conditions as the select tones.
 5. The method of claim 4 whereinfor each remaining user, selecting ones of the tones further comprisesminimizing a number of tones pre-assigned as select tones while ensuringa target data rate is achieved in light of the channel conditionsassociated with each of the select tones.
 6. The method of claim 1further comprising: determining a number of the select tones fortransmitting original data and a number of the select tones fortransmitting redundant data; and increasing the number of the selecttones for transmitting redundant data for remaining users with poorchannel conditions.
 7. The method of claim 1 wherein selecting aremaining user further comprises: determining a scheduling factor foreach remaining user based on the channel condition indicia; andselecting the remaining user having the least favorable schedulingfactor as the active user.
 8. The method of claim 1 wherein the datascheduled for transmission is real-time data.
 9. The method of claim 1wherein the data scheduled for transmission is voice information. 10.The method of claim 1 wherein groups of the tones with a time andfrequency continuum associated with the transmit time interval areassociated with channels, and the tones are pre-assigned to theremaining users and permanently assigned to the active users accordingto corresponding channels.
 11. The method of claim 1 wherein groups oftones are associated, and further comprising effecting signaling forscheduling based on the groups of tones to reduce signaling overhead.12. The method of claim 1 wherein the number of tones pre-assigned toremaining users increases with each re-transmission attempt.
 13. Themethod of claim 1 wherein the multi-carrier communication environment isan orthogonal frequency division multiplexing (OFDM) communicationenvironment and the select tones are OFDM tones.
 14. A system forscheduling data for transmission during a transmit time interval in amulti-carrier communication environment comprising: a communicationinterface; a network interface; and a control system associated with thecommunication interface and the network interface, the control systemconfigured to: determine channel condition indicia for a plurality ofusers; and in an iterative manner: pre-assigning select tones for'eachremaining user of the plurality of users, which have not beenpermanently assigned tones for the transmit time interval; selecting aremaining user having least favorable channel conditions as an activeuser; and permanently assigning to the active user the select tonespre-assigned to the active user wherein once the select tones arepermanently assigned to the active user, the active user is no longer aremaining user.
 15. The system of claim 14 wherein the select tonespermanently assigned to active users are no longer available forpre-assignment to the remaining users.
 16. The system of claim 14wherein the control system is further configured to initiate schedulingfor the transmit time interval for the plurality of users using theselect tones permanently assigned to each of the plurality of users. 17.The system of claim 14 wherein for each remaining user, to pre-assignthe select tones, the control system is further configured to: sorttones in light of channel condition information; and select ones of thetones having most favorable channel conditions as the select tones. 18.The system of claim 17 wherein for each remaining user, to select onesof the tones, the control system is further configured to minimize anumber of tones pre-assigned as select tones while ensuring a targetdata rate is achieved in light of the channel conditions associated witheach of the select tones.
 19. The system of claim 14 wherein the controlsystem is further configured to: determine a number of the select tonesfor transmitting original data and a number of the select tones fortransmitting redundant data; and increase the number of the select tonesfor transmitting redundant data for remaining users with poor channelconditions.
 20. The system of claim 14 wherein to select a remaininguser, the control system is further configured to: determine ascheduling factor for each remaining user based on the channel conditionindicia; and select the remaining user having the least favorablescheduling factor as the active user.
 21. The system of claim 14 whereinthe data scheduled for transmission is real-time data.
 22. The system ofclaim 14 wherein the data scheduled for transmission is voiceinformation.
 23. The system of claim 14 wherein groups of the tones witha time and frequency continuum associated with the transmit timeinterval are associated with channels, and the tones are pre-assigned tothe remaining users and permanently assigned to the active usersaccording to corresponding channels.
 24. The system of claim 14 whereingroups of tones are associated, and further comprising effectingsignaling for scheduling based on the groups of tones to reducesignaling overhead.
 25. The system of claim 14 wherein the number oftones pre-assigned to remaining users increases with eachre-transmission attempt.
 26. The system of claim 14 wherein themulti-carrier communication environment is an orthogonal frequencydivision multiplexing (OFDM) communication environment, and the tonesare OFDM tones.